1. Field of the Invention
The present invention relates to an image forming device including: a high voltage generating circuit for applying an oscillating voltage to a charging member disposed in contact with or proximity to an image carrier, the oscillating voltage having a DC voltage and an AC voltage superimposed thereon; a current detecting portion for detecting the value of a DC current between the image carrier and the charging member; and an AC voltage control portion for controlling the value of the AC voltage so as to secure the detected value of the DC current within a target current range.
2. Description of the Related Art
Charging control devices mounted in recent image forming devices are dominantly of the contact charging system, where a charging member in the form of a roller, a blade, and the like is disposed in contact with or proximity to an image carrier surface, and the charging member is applied an oscillating voltage having a DC voltage and an AC voltage superimposed thereon, thereby uniformly charging the image carrier surface. The contact charging system is in the mainstream because of considerations including the voltage reducing process of reducing the charging control voltage to the image carrier, a reduction in ozone that occurs during charging control, and cost reduction. The oscillating voltage is not limited to a sine wave, but any periodically changing, oscillating wave may be used such as a rectangular wave, a triangular wave, and a pulse wave.
Japanese Unexamined Patent Publication No. 63-149668 discloses charging properties of the contact charging system including the following:
If the AC voltage of the oscillating voltage increases its peak-to-peak voltage, then the charge voltage of the image carrier increases in proportion thereto; if the peak-to-peak voltage reaches a level that is substantially twice the voltage at which the charging starts with the DC voltage, then the charge potential is saturated, and no further voltage increase will change the charge potential; in order to secure charge uniformity, the peak-to-peak voltage of the applied oscillating voltage must be at least twice the voltage at which the charging starts with application of the DC voltage, which is determined by, for example, the properties of the image carrier; and the obtained charge voltage depends on the DC component of the applied voltage.
Japanese Unexamined Patent Publication No. 2001-201921 discloses a technique to prevent problems including deterioration of the image carrier, toner fusing, and image deletion, by securing a constant amount of discharge by preventing excessive discharge irrespective of manufacture variations of resistance of charging members and environment-caused resistance fluctuations.
Specifically, the reference discloses a charge control method that determines the peak-to-peak voltage of the AC voltage applied to the charging member during image formation, on the basis of AC current values measured during a period of time other than image formation, the AC current values including the value of the AC current through the charging member when the charging member has been applied one or more peak-to-peak voltage of less than twice the discharge starting voltage Vth, and the values of the AC current through the charging member when the charging member has been applied two or more peak-to-peak voltages of equal to or more than twice the discharge starting voltage Vth. As used herein, the discharge starting voltage Vth refers to a discharge starting voltage to the image carrier at the time when a DC voltage is applied to the charging member.
More specifically, constant voltage control is carried out such that a peak-to-peak voltage securing fI2 (Vpp)−fI1 (Vpp)=D is determined, where fI1 (Vpp) denotes a function of peak-to-peak voltage and AC current plotted as a line connecting 0 and an AC current value when the charging member is applied one peak-to-peak voltage of less than twice Vth, and fI2 (Vpp) denotes a function of peak-to-peak voltage and AC current plotted as a line connecting AC current values when the charging member is applied two or more peak-to-peak voltages of equal to or more than twice Vth. Then the determined peak-to-peak voltage is rendered a constant peak-to-peak voltage for the AC voltage applied to the charging member during image formation. Note that D is a predetermined constant.
However, it has been found that the technique recited in the JP2001-201921 publication poses the following problems when applied to, for example, an image carrier of 30 mm in diameter with a photoreceptor layer of 20 μm thick amorphous silicon and a charging member made of a charging roller of epichlorohydrin rubber that is contact with the image carrier at a pressure of 1 kgf.
In high-temperature and high-humidity environments, where the electrical resistance of the charging member is relatively low, a desired discharge current value D can be obtained from AC current value properties obtained by the above-described method, whereas in normal-temperature and normal-humidity environments and in low-temperature and low-humidity environments, it is difficult to obtain a desired discharge current value D on the basis of AC current value properties obtained by the above-described method.
In view of this, the Applicants of this application proposed a technique to control the charge potential of the image carrier irrespective of environmental fluctuations such as in temperature and humidity and aging of the image carrier, the charging member, and the like (Japanese Unexamined Patent Publication No. 2007-199374).
Specifically, the Applicants disclosed a charge control device for controlling the output voltage of a high voltage generating circuit for applying an oscillating voltage to a charging member disposed in contact with or proximity to an image carrier, the oscillating voltage having a DC voltage and an AC voltage superimposed thereon, the charge control device including current detecting means of detecting the value of a DC current Idc between the charging material and the image carrier, and DC voltage control means of controlling the DC voltage so as to secure the value of the DC current Idc detected by the current detecting means within a target current range.
When an oscillating voltage is applied to the charging member, and the value of the DC current Idc between the image carrier and the charging material is measured, the DC current Idc and charge potential Vo are proportional to one another, as shown in FIG. 12A, and this relation is proven to show no substantial change due to environmental fluctuations such as in temperature and humidity and aging of the image carrier, the charging member, and the like.
Thus, adjusting the DC current Idc can set the charge potential Vo at a target potential.
The JP63-149668 publication states that when the DC voltage Vdc is equal to or more than the discharge starting voltage Vth, the charge potential Vo is a linear function of the DC voltage Vdc.
On the basis of this statement and the above-described proportional relationship between the DC current Idc and the charge potential Vo, the DC current value Idc is a linear function of the DC voltage Vdc when the DC voltage Vdc is equal to or more than the discharge starting voltage Vth, as shown in FIG. 12B.
The JP63-149668 publication further states that controlling the peak-to-peak voltage of the AC voltage at not less than twice the discharge starting voltage Vth stabilizes the charge potential Vo while the charge potential Vo depends on the applied DC voltage Vdc.
Thus, the DC current Idc can be stabilized at a predetermined current level by setting the DC voltage Vdc at a voltage level (equal to or more than the discharge starting voltage Vth) corresponding to a predetermined charge potential Vo and adjusting the peak-to-peak voltage of the AC voltage to approximately twice the discharge starting voltage Vth, thereby making it possible to set the charge potential of the image carrier at a predetermined, stable charge potential Vo while preventing discharge products that occur due to application of an excessive level of AC voltage.
However, referring to FIG. 13, when epichlorohydrin rubber is employed for the material of the charging member, in a low-temperature environment (low-temperature environment 1 in FIG. 13), the conductive ions inside the epichlorohydrin rubber move slowly resulting in high electrical resistance. In order to adjust the charge potential of the image carrier to the stable target potential, the peak-to-peak voltage of the AC voltage must be controlled at a peak-to-peak voltage Vpp2, which is higher than the peak-to-peak voltage Vpp1 of a normal-temperature environment.
In particular, in a low temperature as extreme as 0° C. (low-temperature environment 2 in FIG. 13), the charge potential of the image carrier cannot be adjusted to the target potential no matter how high the peak-to-peak voltage is made, causing the problem of fog and darkness variations occurring on the formed images.
In view of these circumstances, the present inventors made an attempt to promote the movement of the conductive ions inside the epichlorohydrin rubber and thus lower resistance by executing aging control of performing rotation driving of the image carrier while keeping the AC voltage and the DC voltage applied to the charging member at preset voltage values.
However, the present inventors faced the fact that the aging effect is influenced by the frequency of the AC voltage; when an AC voltage of high frequency is applied, the movement of the conductive ions inside the epichlorohydrin rubber cannot follow the high frequency, resulting in an insufficient aging effect.
In particular, in high speed image forming devices, in order to sufficiently charge the image carrier rotating at high speed, the AC voltage must be set at a high frequency to secure an amount of charge per unit length of the image carrier in its circumferential direction. However, as described above, if the frequency of the AC voltage increases, a sufficient aging effect cannot be obtained at low temperature, making it difficult to overcome the problem of fog and darkness variations occurring on the resulting images.